Heat-actuated safety indicator for hotplates and the like



Feb. 10, .1970 R M OR em 7 f 3,494,321

HEAT-ACTUATED SAFETY INDICATOR FOR HOTPLATES AND THE LIKE Filed June 17, 1968 United States Patent US. Cl. 116114.5 3 Claims ABSTRACT OF THE DISCLOSURE A hotplate is provided with a heat-actuated safety device for generating a visual signal when the hotplate is dangerously hot whether or not heat energy is being supplied to the hotplate. A flag is mounted on an extension of a bimetal which is located in a housing attached to a rear wall of the hotplate. When the hotplate is at room temperature, the flag is hidden behind it; and when it is hot, the flag is rotated into plain view above the support surface of the hotplate. The central portion of the bimetal is mounted on a shaft which extends through the housing and is frictionally coupled to it. A slot is provided on the exposed end of the shaft for initial adjustment or calibration of the bimetal at room temperature; and the adjustment is facilitated by a clutchtype mounting securing it to the housing.

The present invention relates to a visual indicator for signaling that the temperature of a heated plate is too high to touch by hand or unsafe to inflammable or low-flashpoint material. More particularly, the present invention relates to a heat-actuated safety indicator which operates independent of the application of energy to the heating element.

Although it will be obvious to persons skilled in the art that it is not so limited, the safety indicator herein described is particularly suited for use with laboratory hotplates which typically are heated with electrical energy. In some electrically-energized hotplates, a visual indicator, such as a constant red light, is actuated during the period in which the hotplate is being heated. However, when the power is turned oif, the indicator light is also turned off. Thus, should another person desire to use the hotplate, he is not warned that the hotplate is at too high a temperature to be touched.

The problem is somewhat more acute in the laboratory wherein the contents of a vessel are required to be subjected to a particular heat for a specified time, after which the vessel is removed and its contents are subjected to another process thus leaving an unattended hotplate at a high temperature.

Summary The present invention provides a heat-actuated safety device which is mounted right on the hotplate. As used herein, the word hotplate does not refer to the overall heater apparatus; but rather, it refers specifically to the heat-conductive plate having a surface through which heat is transmitted from an energy source to an object or vessel and which also serves as a support surface for the vessel.

A cup-shaped housing has a peripheral flange about its open end for mounting directly to the hotplate. A shaft extends coaxial with the cylindrical sidewall of the housing and protrudes through a central aperture in the back- "Ice wall thereof. The shaft is mounted on this backwall by means of a clutch type of coupling which permits rotation of the shaft within the housing to permit calibration, but secures the shaft in fixed angular position once the adjustment is made. A slot extends circumferentially about the housing sidewall for about 60.

The exposed end of the central shaft is provided with a slot for receiving a screwdriverrand interior of the housing, a bimetal ribbon is mounted on the shaft. An extension arm of the bimetal ribbon extends through a slot in the cylindrical sidewall of the housing. At the distal end of the extension arm, an indicia-bearing flag is mounted.

The slot in the sidewall extends from the horizontal upwards to encompass an arc of about 60 therealong. An initial adjustment is made when the hotplate is at room temperature by rotating the shaft by means of a screwdriver in the exposed rear slot so that the extension arm lies just along the horizontal edge of the slot in the housing without any induced stress. As the temperature of the hotplate rises, the bimetal deflects to rotate the extension arm thereof upwardly and to raise the indicia-bearing flag above the supporting surface of the hotplate thereby generating a visual indication that the same is at a temperature unsafe for touching. Thus, even though power may be shut off and the operator leaves the hotplate, another person desiring to use it will have an immediate indication that it is hot since the flag does not return to its hidden position until the hotplate returns to near room temperature.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein like reference numerals will refer to identical parts in the various views.

The drawing FIG. 1 is a perspective view of a conventional hotplate equipped with the safety device of the present invention;

FIG. 2 is a rear elevation view of the inventive device;

FIG. 3 is a closeup front elevation view of the inventive device illustrating the range of positions of movement of the flag;

FIG. 4 is a top view of the device of FIG. 3; and

FIG. 5 is a perspective view of the shaft on which the bimetal ribbon is mounted.

Detailed description Referring first to FIG. 1, there is seen a conventional laboratory heater generally designated 10; and it is equipped with an electrical heater element (not shown) which may be located either within or beneath a hotplate 11. The temperature of the hotplate 11 is automatically regulated according to known techniques; and it is set by means of a front dial 12. The hotpla-Tth 11 serves not only as a heat-conductive surface for transmitting heat from the heater element to the vessel or container of the substance being heated, but it also serves as a support surface for that vessel.

For laboratory use, the heater 10 may also be equipped with a magnetic stirrer, which is also conventional and therefore not shown. The speed of the stirrer may be set by means of a dial 14; and a Teflon-coated magnetic stirrer may be provided for insertion to the liquid in a container supported by the hotplate 11.

The inventive safety device is generally designated by the reference numeral 15 in FIG. 1; and as illustrated there, an indicia-bearing flag- 16iis in. a raised or.ob-.

servable condition. The flap 16 preferably bears the indicia HOT; and it is made of anodized aluminum and colored bright red for signaling danger. The actual indicia HOT may be etched into the aluminum or painted in white or the red background.

Turning now to FIG. 2-4, the heat-actuated safety device includes an integral housing generally designated 18. The housing 18 includes a cylindrical sidewall 19 having a peripheral mounting flange 20, and a backplate 27. The flange 20 is provided with first and second apertures 21 and 21a (FIG. 3) for receiving screws 22 and 23 respectively (FIG. 2) which are threadably received in a rear vertical surface 11a of the hotplate 11.

The entire housing 18, as mentioned, is integral, and is of a material having good heat-conductive properties; we have found aluminum to give satisfactory results. The mounting flange 20 also provides a good heat-conductive surface in engagement with the hotplate for maintaining the entire housing at nearly the same temperature as the hotplate. A slot 25 (FIG. 4) is formed in the cylindrical sidewall 19 of the housing 18; and it extends from a horizontal edge 26 upwardly about 60 around the cylindrical sidewall 19. The backplate 27 together with the cylindrical sidewall 19 form a cup-shaped container for housing a bimetal presently to be described.

The backplate 27 defines a central aperture 28 for receiving a shaft generally designated 29 and seen in perspective in FIG. 5. The shaft 29 contains an enlarged head portion 30 and a partially-threaded shank 31 which fits through the aperture 28. A shoulder 32 extends between the shank 31 and the head portion 30 of the shaft 29; and it engages the interior surface of the backplate 27 when attached thereto (see FIG. 4). The aperture 28 of the backplate 27 is large enough to receive the shank 31 of the shaft 29; however, it has a smaller diameter than that of the head portion 30 of the shaft 29. The shank 31 is provided with a machine screw thread which terminates short of the shoulder 32 to provide a smooth bearing surface 31a for engaging the wall of the aperture 28 and for permitting rotation of the shaft 29 as presently described.

The shaft 29 is secured to the housing by means of a nut 33 threadably received on the threaded portion 31 of the shaft. A flat washer 33a and a spring washer 33b are interposed between the nut 33 and the backplate 27 (FIG. 4). When the nut 33 is threaded into the portion 31 of shaft 29, the flat washer 33a and the shoulder 32 frictionally engage the backplate 27 of the housing 18 with a preset force which must be overcome before the shaft 29 can be rotated relative to the housing. However, once it is rotated, the shaft 29 and its securing nut 33 rotate together by virtue of the clutch action provided .by the spring washer 33b. This is considered an important aspect of the present invention in that it permits initial adjustment or calibration of the bimetal described in greater detail below.

The end of the threaded portion 31 of the shaft 29 is provided with a slot 35 for receiving a screwdriver or other blade in adjusting the angular position of the bimetal. The head portion 30 of the shaft 29 is provided with a central slot 36.

A bimetal ribbon 38, preferably in the foam of a spiral, is mounted about the head portion 30 of the shaft 29 within the housing 18; and it is secured to the shaft 29 by means of a transversely-bent end portion 39 which is received in the slot 36 of the head 30 and held there by friction.

As seen in FIG. 3, which is a front view, the ribbon 38 is wound clockwise about the shaft 29; and as is conventional, it is comprised of two metals having different thermal coeflicients of expansion and placed side by side throughout its length. The bimetal 38 is also provided at its free end with an extension arm 41 which protrudes radially outward of the shaft 29 and projects through the circumferentialslot 25 in the cylindrical sidewall.19 of the housing 18. At the distal end of the extension arm 41, the indicia-bearing flag 16 is mounted by means of rivets 43 and 44. i

For the arrangement shown, the metal on the outside of the bimetal winding, as indicated by reference numeral 45, has a smaller thermal coeflicient of expansion than does the metal on the interior. Thus, as the ribbon is heated, the interior metal will unwind the spiral and rotate the extension arm 41 in a counterclockwise direction as indicated by the dashed arrow 48 in FIG. 3 thereby raising the flag 16 to the dashed portion illustrated in FIG. 3. By thus rotating the arm 41 in a counterclockwise direction, the flag is lifted above the supporting surface of the hotplate 11 where it will be clearly visible from the front.

It is noted that the flag 16 will remain in such raised condition (thereby displaying the warning indicia on it) until the temperature of the hotplate is lowered to near room temperature. As illustrated in FIG. 3 in dotted line, additional lifting is supplied to the flag 16 since the entire arm 41 is also comprised of the bimetal arrangement; and when heated, the arm 41 will curve in the direction of rotation of the flag which supplies a cumulative displacement thereof together with the deflection caused by the unwinding of the coil.

In order to adjust the safety device for proper operation, a screwdriver is inserted in the exposed rear slot 35 on the threaded portion 31 of the shaft 29; and the shaft 29 is thus rotated until the extension arm 41 is raised above the horizontal edge 26 of the slot 25 thereby indicating that all stress had been removed from the bimetal 38. Thence, the arm 41 is rotated slightly clockwise (when viewed in FIG. 3) until the arm 41 barely engages the horizontal edge 26 of the slot 25. This operation is performed with the nut 33 in its tightened condition; and it is facilitated by the previously-described clutch action provided by the spring washer 33b. Hence, once the adjustment is made, the shaft and the bimetal remain in frictional coupling with the housing; and no further adjustment is required for proper operation.

It will be apparent that the motion of the arm could be calibrated to indicate an analog of temperature in which case the advantageous adjustment arrangement would provide a good means of initial calibration.

Having thus described in detail a preferred embodiment of the inventive safety device, it will be obvious to persons skilled in the art that certain modifications may be made to the structure disclosed by way of example, and that equivalent materials may be substituted. It is our intention that all such modifications and substitutions be covered.

We claim: p

1. In combination with a hotplate, a heat-actuated safety device comprising: a heat-conductive housing having a peripheral mounting flange secured in heat-conductive relation to said hotplate, said housing defining a slot; a shaft mounted in said housing; a bimetal ribbon mounted on said shaft and having an arm extending through said slot and moving between a first position wherein said arm lies generally behind said hotplate when said ribbon is at room temperature and a second position wherein said arm extends above said hotplate when said hotplate is hot; and an indicia-bearing flag mounted on the distal end of said arm, whereby as the temperature of said bimetal rises, the same will unwind thereby raising the flag above the supporting surface of said hotplate and displaying the same.

2.'The structure of claim 1 wherein said hotplate defines a horizontal supporting surface and wherein said housing includes a cylindrical sidewall, a backpla-te integral'with said sidewall and said peripheral mounting flange secured to said hotplate, said slot extending upwardly therefrom about 607; said arm of said bimetal extending generally horizontally through said slot in said first position when said hotplate i at room temperature.

3. The structure of claim 2 wherein said backplate of said housing defines a central aperture for receiving said shaft, said shaft including an expanded head portion defining a shoulder engaging the interior surface of said backplate, said head further defining a reduced threaded portion extending through said aperture and defining a slot exterior of said housing; said device further including a nut threaded onto said threaded portion of said shaft; and a spring washer interposed between said nut and said backplate whereby a blade may be inserted into the exposed slot of said shaft for adjusting the angular position of said extension arm at room temperature to calibrate the same while a clutching action provided by said washer prevents relative movement between said 15 Securing nut and said shaft during said adjustment,

References Cited UNITED STATES PATENTS 1,622,545 3/1927 Schellenberger 73-3635 XR 2,750,916 6/ 1956 Hanington 116-102 3,163,043 12/1964 Snider 73-3635 3,301,060 l/l967 Kenrnotsu 73-343 LOUIS R. PRINCE, Primary Examiner 10 FREDERlCK SHOON, Assistant Examiner U.S. C1. X.R. 

